A typical semiconductor laser has two parallel cleaved faces, forming between them an optical cavity. Due to the difference in refractive indices of the semiconductor and air, the faces form partially reflecting mirrors which act as a feedback mechanism. Light is emitted parallel to the p-n junction through the cleaved faces.
Burnham et al., in U.S. Pat. No. 4,302,729 describe a distributed feedback laser that has periodic corrugations on the top surface of the active layer lying above the p-n junction. These corrugations eliminate the need for cleaved mirror surfaces and have the additional advantage of wavelength selectivity. However, the laser emits light in a direction parallel to the p-n junction making the construction of a two-dimensional array of lasers extremely difficult.
McGroddy, in U.S. Pat. No. 3,996,492 and Springthorpe et al. in U.S. Pat. No. 4,163,953 show lasers that emit light in a direction perpendicular to the active layer. This is accomplished by bending the optical cavity with total internal reflecting surfaces oriented 45.degree. with respect to the active layer. These surfaces are the faces of grooves etched into the laser surface Light is emitted through a hole etched into the substrate with the base of the hole acting as the partially reflecting feedback means. The drawback of this arrangement is mentioned briefly in Springthorpe, A. J., "A Novel Double-heterostructure p-n-junction Laser," Appl. Phys. Lett. Vol. 31, No. 8, pp 524-5, 15 Oct. 1977. The beam diverges after it is bent out of the plane of the active layer, so only a small fraction of light returns to the junction plane for further amplification. This increases the needed threshold current, reducing efficiency and power output, and causing thermal problems.